<Window x:Class="WpfApplication42.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <Style TargetType="{x:Type StackPanel}">
            <Setter Property="Margin" Value="3"/>
        </Style>
        <Style TargetType="{x:Type Button}">
            <Setter Property="Margin" Value="3"/>
            <Setter Property="Padding" Value="10,3"/>
        </Style>
    </Window.Resources>
    <DockPanel>
        <StackPanel Orientation="Horizontal" DockPanel.Dock="Top">
            <Button Content="Add UserControl1"
                    Click="AddUserControl1_OnClick"/>
            <Button Content="Add UserControl2"
                    Click="AddUserControl2_OnClick"/>
        </StackPanel>
        <StackPanel x:Name="MyPanel" Orientation="Vertical"/>
    </DockPanel>
</Window>

using System.Diagnostics;
using System.Windows;

namespace WpfApplication42
{
    public partial class MainWindow : Window
    {

        public static readonly RoutedEvent UserControlLoadedEvent = EventManager.RegisterRoutedEvent("UserControlLoaded", RoutingStrategy.Bubble, typeof(RoutedEventHandler), typeof(MainWindow));

        public MainWindow()
        {
            InitializeComponent();
            AddHandler(UserControlLoadedEvent, new RoutedEventHandler(OnUserControlLoaded), true);
        }

        private void OnUserControlLoaded(object sender, RoutedEventArgs e)
        {
            Debug.Print(e.OriginalSource.ToString());
        }


        private void AddUserControl1_OnClick(object sender, RoutedEventArgs e)
        {
            MyPanel.Children.Add(new UserControl1());
        }

        private void AddUserControl2_OnClick(object sender, RoutedEventArgs e)
        {
            MyPanel.Children.Add(new UserControl2());
        }
    }
}

<UserControl x:Class="WpfApplication42.UserControl1"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
        <Border Background="BlanchedAlmond" MinHeight="50"/>
</UserControl>

using System.Windows;
using System.Windows.Controls;

namespace WpfApplication42
{
    public partial class UserControl1 : UserControl
    {
        public UserControl1()
        {
            InitializeComponent();
            Loaded += UserControl1_Loaded;
        }

        void UserControl1_Loaded(object sender, RoutedEventArgs e)
        {
            RaiseEvent(new RoutedEventArgs(MainWindow.UserControlLoadedEvent, this));
        }
    }
}

<UserControl x:Class="WpfApplication42.UserControl2"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
        <Border Background="LightBlue" MinHeight="50"/>
</UserControl>

using System.Windows;
using System.Windows.Controls;

namespace WpfApplication42
{
    public partial class UserControl2 : UserControl
    {
        public UserControl2()
        {
            InitializeComponent();
            Loaded += UserControl2_Loaded;
        }

        void UserControl2_Loaded(object sender, RoutedEventArgs e)
        {
            RaiseEvent(new RoutedEventArgs(MainWindow.UserControlLoadedEvent, this));
        }
    }
}